• Be ≥18 years of age (or the higher legal age in the jurisdiction in which the study is taking place) at the time of informed consent

• Has documented diagnosis of MM according to the IMWG diagnostic criteria (Rajkumar 2011).

• Has received 2 or more prior MM therapies including a PI, IMiD and CD38 antibody.

• Has an ECOG performance status (Oken 1982) of 0 to 1.

• Measurable disease at screening, as assessed by local laboratory, defined by any of the following:

‣ Serum M-protein level ≥0.5 g/dL; or

⁃ Urine M-protein level ≥200 mg/24 hours; or

⁃ Light chain MM without measurable M-protein in the serum or the urine: serum free light chain (sFLC) ≥10 mg/dL and abnormal serum immunoglobulin kappa lambda FLC ratio.

⁃ For participants without measurable disease in the serum, urine, or involved FLC, presence of plasmacytomas (≥2 cm).

• Human immunodeficiency virus-positive participants are eligible if they meet all of the following:

‣ No detectable viral load (i.e., \<50 copies/mL) at screening

⁃ CD4+ count \>300 cells/mm3 at screening

⁃ No acquired immunodeficiency syndrome (AIDS)-defining opportunistic infection within 6 months of screening

⁃ Receiving highly active antiretroviral therapy (HAART). Any changes in HAART due to resistance/progression should occur at least 3 months prior to enrollment. A change in HAART due to toxicity is allowed up to 4 weeks prior to enrollment.

• Adequate organ system function

• Body weight \>35 kg.

• A participant of childbearing potential must have a negative highly sensitive serum (β-hCG) at screening and within 72 hours of the start of study treatment and must agree to further serum or urine pregnancy tests during the study.

• A participant must agree to abide by protocol defined contraceptive requirements for the duration of the study

• A participant must sign an ICF indicating that participant understands the purpose of, and procedures required for, the study and is willing to participate in the study.

• A participant is required to stay within 30 minutes of transportation to the site and remain in the company of a competent adult at all times until 48 hours following administration of all doses within the teclistamab or talquetamab step-up dosing schedule

• A participant must agree to carry the study participant identification wallet card at all times.

• A participant must comply with all the protocol requirement procedures, including measuring and recording of body temperature and blood oxygen saturation twice daily (≥8 hours apart) during the first 2 cycles of teclistamab or talquetamab treatment and coming to the study site for safety assessments.

• A participant and the accompanying competent adult must be made aware of the presenting sign sand symptoms of teclistamab- or talquetamab- associated toxicities, including but not limited to CRS, ICANS, infections, etc. The accompanying competent adult must watch the participant at all times for teclistamab- or talquetamab- associated toxicities, until 48 hours after the first treatment dose of teclistamab or talquetamab.